Perhaps my previous calculations of power consumption are too simplistic.
In researching green options, I discovered on some forums the use of a 5 port USB powered hub to deliver power to the RPi, and its peripherals, with the RPi delivering communications and data back to the hub. I had been wondering about the concept of using one hub to power multiple RPis - would it be more efficient and greener?
Both the RPi and the hub run from their own transformers. The RPi transformer has an input of 240V at 50-60Hz, drawing 150mA, and delivers a DC output of 1A 5VA. To my limited knowledge of the calculations involved, this means that the input power is: 240 x 0.15 = 36 Watts, and the output is: 5 x 1 = 5 Watts. This would imply that 31 Watts is lost to the resistance and impedance of the unit, and generates heat in the transformer.
When I look at the input and output of the USB hub's transformer, the input is: 240V at 50-60Hz, drawing 1A, therefore 240 Watts. The output is 5V 1.5 A, therefore 7.5 Watts.
This would imply that whilst the powered hub supplied 50% more power, it draws 660% more power from the mains, and hence would not be a greener more efficient supply.
However, I know that my knowledge in this area is presently limited - am I using the right calculations and making the right deductions? Should I be calculating and comparing the power consumption of the transformers from their input or output values? Are these constant loads, or the maximum loads of each unit?
Anyone able to help?
Just as a followup - called into one of the local Jaycar stores. Spoke with the friendly staff who advised that I was on the right track with using the input specifications for determining the power draw.
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